Two helicopters that ditched in the North Sea in separate incidents five months apart experienced a loss of oil pressure caused by failure of part of the main rotor gearbox, investigators have concluded.

Analysis of the 2012 incidents by the Air Accidents Investigation Branch (AAIB) found that both aircraft had a crack in the vertical gearbox shaft that drives the oil pumps.

The loss of oil pressure required crews to activate an emergency lubrication system, the report found. But shortly after doing so, a warning light came on that suggested the backup system had failed and demanded they ditch the helicopters immediately.

Officials have made 10 safety recommendations in the wake of both incidents.

No lives were lost and no one was seriously injured as a result of the two incidents examined in the report, which is published on Wednesday.

The first incident happened on 10 May 2012. Twelve passengers and two crew on board a Super Puma destined for an oil platform were rescued after ditching about 40 miles east of Aberdeen.

On 22 October, another Super Puma carrying an oil crew from Aberdeen to a rig north of Shetland was forced to ditch about 35 miles south-west of Sumburgh. The 17 passengers and two crew were rescued from liferafts by a passing vessel.

Because of the similarities between the two incidents, the AAIB decided to combine its investigations into the ditchings in a bid to discover the root causes of what happened.

Experts concluded that there were two "causal factors" relating to the ditching of both helicopters.

The first was what the AAIB described as a "360-degree circumferential high-cycle fatigue crack" which it said "led to the failure of the main gearbox bevel gear vertical shaft and loss of drive to the oil pumps".

A further factor was what it described as incompatibility between the aircraft wiring and the internal configuration of pressure switches, causing the warning light for the backup lubrication system to come on. The AAIB found that in both accidents the emergency lubrication system, once activated, appeared to have worked successfully.

Three factors contributed to the failure of the helicopters' gearbox shafts, the report found. These were that the maximum stress in an area of welding where the cracks occurred had been underestimated, that residual stresses introduced during the welding work had not been fully taken into account during the design of the shaft, and that some corrosion had been present on both shafts.

Summarising the incidents, the report said: "While operating over the North Sea, in daylight, the crews of G-REDW (involved in the May incident) and G-CHCN (the Sumburgh ditching) experienced a loss of main rotor gearbox oil pressure, which required them to activate the emergency lubrication system.

"This system uses a mixture of glycol and water to provide 30 minutes of alternative cooling and lubrication. Both helicopters should have been able to fly to the nearest airport; however, shortly after the system had activated, a warning illuminated indicating that the emergency lubrication system had failed.

"This required the crews to ditch their helicopters immediately in the North Sea. Both ditchings were successful and the crew and passengers evacuated into the helicopter's liferafts before being rescued. There were no serious injuries.

"The loss of oil pressure on both helicopters was caused by a failure of the bevel gear vertical shaft in the main rotor gearbox, which drives the oil pumps. The shafts had failed as a result of a circumferential fatigue crack in the area where the two parts of the shaft are welded together."

It went on: "The emergency lubrication system operated in both cases, but the system warning light illuminated as a result of an incompatibility between the helicopter wiring and the pressure switches. This meant the warning light would always illuminate after the crew activated the emergency lubrication system."

AAIB officials said a number of other safety issues had been identified, concerning emergency checklists, the crash position indicator and liferafts.

The helicopter involved in the May incident was operated by Bond Offshore Helicopters, while the aircraft in the October incident was operated by CHC Scotia , the report noted.

The branch has issued 10 safety recommendations for manufacturers and aviation officials in the wake of the incidents.

The helicopter manufacturer, Eurocopter, has carried out several safety actions and is redesigning the bevel gear vertical shaft in light of the investigation findings, the AAIB said.